1. Field of the Invention
The present invention is directed to a process for cleaning zirconium-niobium alloys which are used for cladding of fuel in thermal reactors. More particularly, the present invention is directed to a rapid, efficient and complete chemical process for removal of surface second-phase particle deposits from zirconium-niobium alloys, in which a clean, shiny, non-pitted surface is obtained without the need to use water blasting or mechanical wiping operations.
2. Description of the Prior Art
Zirconium (Zr) has many useful properties, among them good physical strength and high corrosion resistance. In its hafnium-purified form, zirconium is widely used as the structural material for nuclear fuel cores, taking advantage of its low neutron absorption cross section. Current alloys used in nuclear grade zirconium applications typically contain tin, iron and sometimes nickel; more recent alloy compositions such as the “Zirlo” alloy (Westinghouse Electric Co., LLC, Pittsburgh, Pa.) also contain percent quantities of niobium (Nb) for improved corrosion resistance in nuclear reactor environments.
Like most metal product applications, fabrication of nuclear reactor fuel tubes and core components leaves the metal with undesirable surface features such as scratches, oxidation stains and chemical contamination from lubricants. Zirconium components therefore are pickled before use and the parts making up the fuel assemblies may be pickled numerous times during the manufacturing process to control the surface quality and remove contaminants. A typical pickling bath for zirconium contains between 10 to 40% weight nitric acid and 1 to 5% hydrofluoric acid, which is a very aggressive solution.
A specific problem arises when pickling niobium-containing zirconium alloys. The niobium tends to segregate within the alloy into very small particles, referred to as second-phase particles (SPPs). The SPPs typically have binary Zr—Nb or ternary Zr—Nb—Fe compositions. When the alloy is pickled, dissolution of the Zr matrix proceeds faster than that of the SPPs, so that large quantities of extremely fine, black particles are released into the pickle acid during the pickling process. Unfortunately, when the zirconium alloy is removed from the pickle acid, even after thorough rinsing, the surface typically is matte black due to a dense coating of adherent particles that do not release from the metal surface during rinsing. This material is known in the industry as “smut,” a reference to the similarity of its appearance to black masses of fine fungal spores by the same name.
Before any niobium-zirconium alloy can be used in a nuclear reactor application, all of the “smut” deposit must be removed, partially to yield bright, shiny product surfaces, and to prevent later possible release of such particles into the reactor cooling water and potential deposition within the reactor. On easily accessible exterior surfaces, removal of SPP deposits is not difficult and can be accomplished by water blasting or mechanical wiping with cloths or sponges. However, many final reactor components contain internal surfaces that are not easily accessible, such as the interior of fuel tubes for both pressurized water reactors (PWRs) and boiling water reactors (BWRs), and the interior of channel boxes in BWRs. Mechanical cleaning of some interior surfaces such as smooth cylindrical tubes may be accomplished by dragging cleaning swabs (“pigs”) through the component, but other small channels cannot be cleaned effectively, and small crevices cannot be accessed at all.
An ideal solution to the problem would be a chemical wash, in which the component can be dipped, which would either dissolve the SPP deposit or release it from the metal surface. Dissolution of Zr—Nb and Zr—Nb—Fe second phase particles is probably not a likely solution, as any solvent capable of such attack would surely attack the zirconium background even more aggressively, leading to both surface damage and release of still more SPPs from the alloy.
There exists a need, therefore, for a process to quickly and completely remove Zr—Nb and Zr—Nb—Fe second phase particles from the entire surface of zirconium-niobium alloys without damaging the surface of the alloys.